1. Field of the Invention
This invention relates to an optical device and a method for making the same, more particularly to a method using an orienting material having molecules with a molecular structure that is stretchable and that has molecular units and orienting spaces which are capable of orienting rod-like liquid crystal molecules of a liquid crystal material.
2. Description of the Related Art
Conventional compensators, which serve as phase retarders for correcting retardation or phase difference in a liquid crystal cell of a liquid crystal display so as to improve the viewing angle, the contrast, and the grey scale of the liquid crystal display, are commonly made by a polycarbonate film, which is required to be stretched along a machine direction (MD) or film moving direction. As a consequence, the extraordinary axis of the thus formed compensators is parallel to the machine direction. As such, the compensators cannot be attached to a polarizer in a roll-to-roll manner when the extraordinary axis of the compensator is to be formed an angle with an absorbing axis of the polarizer.
Optical compensators made from anisotropical materials of liquid crystal molecules are also known in the art.
The compensators are made by aligning the liquid crystal molecules on a substrate using an orienting film which is rubbed along a direction parallel to the machine direction. As a consequence, the projection of the orientation of the liquid crystal molecules of the thus formed compensator on a plane defined by the substrate is parallel to the film moving direction or the machine direction. Note that the compensators are normally produced in the form of rolls. Since the projection of the orientation of the liquid crystal molecules of a compensatory roll is parallel to the machine direction, the compensatory roll cannot be directly attached to a polarizer roll in a roll-to-roll manner for preparing a polarization element having a function of a quarter wave plate, which can generate a phase difference of λ/4 at a wavelength of 550 nm, or a function of a half wave plate, which can generate a phase difference of λ/2 at a wavelength of 550 nm.
FIG. 1 illustrates a polarization element that includes a polarizer piece 1 and a compensator piece 21 cut from a compensator roll for attaching to the polarizer piece 1. In order to serve as a quarter wave plate, the compensator piece 21 is required to be cut from the compensator roll in such a manner that the projection 211 of the orientation of the liquid crystal molecules of the compensator piece 21 forms an angle of 45 degrees with the lengthwise direction (L) of the compensator piece 21, which is parallel to the absorbing axis 11 of the polarizer piece 1.
FIG. 2 illustrates a polarization element that includes a polarizer piece 1 and first and second compensator pieces 21, 22 that are cut from a compensator roll for attaching to the polarizer piece 1. In order to serve as a quarter wave plate, the first compensator piece 21 is required to be cut from the compensator roll in such a manner that the projection 211 of the orientation of the liquid crystal molecules of the first compensator piece 21 forms an angle of 75 degrees with the lengthwise direction (L1) of the first compensator piece 21, and in order to serve as a half wave plate, the second compensator piece 22 is required to be cut from the compensator roll in such a manner that the projection 221 of the orientation of the liquid crystal molecules of the second compensator piece 22 forms an angle of 15 degrees with the lengthwith direction (L2) of the second compensator piece 22. The lengthwise direction (L1) of the first compensator piece 21 and the lengthwise direction (L2) of the second compensator piece 22 are parallel to the absorbing direction of the polarizer piece 1.
As such, mass production using the aforesaid compensators is difficult to achieve, and a large manpower for assembling the compensator pieces and the polarizer pieces is required.
U.S. Pat. No. 6,262,788 discloses a process for preparing an optical retardation film. The process is capable of permitting mass production of the retardation film in a roll-to-roll manner. The process uses a roller set for conveying a compensator substrate. The compensator substrate is continuously rubbed upon moving in a moving direction for alignment of liquid crystal molecules of a liquid crystal film thereon.
Although, theoretically speaking, the aforesaid process is capable of producing the retardation film with the liquid crystal molecules aligned in a direction that forms an angle ranging from 0 degree to 90 degrees with the machine direction or the lengthwise direction of the substrate, the process is relatively complicated due to the use of the roller set in the manufacturing process, and the quality of the product is difficult to control. Moreover, when a rubbing angle is greater than 45 degrees or smaller than −45 degrees, rubbing parameters during rubbing of the compensatory substrate and tension of the compensatory substrate during conveying of the compensatory substrate are difficult to control.
U.S. Pat. No. 6,531,195 discloses an orienting layer of a copolymer for orienting liquid. crystal molecules of an anisotropical layer thereon for preparing an optical compensatory sheet. The aforesaid orienting layer is disadvantageous in that the orientation of the liquid crystal molecules of the anisotropical layer can only be aligned by the orienting layer in a direction perpendicular to a rubbing direction or the machine direction.
Hence, there is a need to develop an orienting layer that is capable of orienting the liquid crystal molecules of an anisotropical layer of a compensator, which is in the form of a roll, such that the projection of the orientation of the liquid crystal molecules of the anisotropical layer forms an angle ranging from 0 degree to 90 degrees with the machine direction, thereby permitting assembly of the compensator and the polarizer in a roll-to-roll manner.